A known pressure operated siren has a structure that includes a base and an external housing that together forms a chamber to house an internal rotor. The rotor is driven by a driver that is disposed externally of the chamber and powered by compressed gas. Rotation of the rotor by the driver pulls external air into the chamber and expels it out of the housing. The manner in which the air flow is expelled from the housing generates a sound wave forming the alarm of the siren. For these known pressure operated sirens, their performance is limited in one or more of the following areas: intensity of alarm sound and duration of the alarm sound.